Lubricating oils, such as those used in the crankcase of internal combustion engines, are designed to reduce friction, minimize deposit formation, prevent corrosion and wear, and to ensure adequate engine lubrication over a wide temperature range. The American Petroleum Institute ("API"), Society of Automotive Engineers ("SAE"), American Society for Testing and Materials ("ASTM") and the International Lubricants Standardization and Approval Committee ("ILSAC") are the key bodies that define industry requirements for engine lubricant quality, establish classifications and develop test methods to certify that lubricants meet performance requirements. Individual original equipment manufacturers ("OEM's") often establish their own performance requirements that may be more stringent than those of API, SAE, ASTM or ILSAC requirements.
To insure that a lubricant has the appropriate low temperature properties generally three tests must be met. It must meet the requirements for pour point (ASTM D97), TP1 Mini Rotary Viscometer, "TP1MRV", (ASTM D 4648) and Scanning Brookfield Gel Index, "Gel Index", (ASTM D 5133). The pour point is an indication of dispensability at low temperatures while the TP1 MRV and Gel Index are measurements of lubricant pumpability at low temperatures.
Additives are used to formulate oil compositions that will meet the foregoing low temperature requirements. Unfortunately experience has shown that the TP1 MRV and Gel Index can vary independently of each other and that adjusting a lubricant composition to meet one of these properties often imposes a negative or undesirable effect on the other property.
Currently the industry requirements for a lubricant Gel Index is a maximum of 12 while the requirements for a lubricant TP1 MRV is a maximum of 60,000 cP at test temperature ranging from -10.degree. C. to -40.degree. C. depending upon the SAE grade. For example, a 5W-30 oil is tested at -35.degree. C., 10W-30 at -30.degree. C., and the like.
OEM's and lubricant formulators, of course, are continually seeking lubricant compositions that have properties that are better than the present standards. Thus, there remains a need for lubricating oils having reduced Gel Index values which still meet TP1 MRV and pour point requirements.